Update 3: 3D Realms webmaster Joe Siegler has commented on the shut down, stating: "It's not a marketing thing. It's true. I have nothing further to say at this time."
Siegler's post also reflects the unexpected nature of the situation, as he was unaware of the impending shut down during an interview conducted yesterday afternoon.
Update 2: Duke Nukem Forever publisher Take-Two has confirmed to Shacknews that it was not funding ongoing development of the 3D Realms project.

"We can confirm that our relationship with 3D Realms for Duke Nukem Forever was a publishing arrangement, which did not include ongoing funds for development of the title," said Take-Two VP of communications Alan Lewis in a prepared statement.

Update: Apogee Software LLC and Deep Silver have issued a statement to Shacknews, confirming that the situation at 3D Realms has not affected the development of Frontline Games' upcoming handheld Duke Nukem Trilogy.
"Deep Silver and Apogee Software are not affected by the situation at 3D Realms," a representative for the companies told us. "Development on the Duke Nukem Trilogy is continuing as planned."

Original story: A very reliable source close to Duke Nukem Forever developer 3D Realms today confirmed to Shacknews that the development studio has shut down.
The closure came about as a result of funding issues, our source explained, with the shut down said to affect both 3D Realms and the recently resurrected Apogee. Employees of both entities have already been let go.

Phone calls and e-mails to various 3D Realms veterans have thus far gone unanswered, with 3D Realms publishing partner Take-Two and Apogee partner Deep Silver likewise unavailable for comment. One 3D Realms spokesperson declined to comment when reached.
3D Realms was founded in 1987 by Scott Miller and George Broussard, and was best known for its Duke Nukem series of shooters starring the titular, bubblegum-lacking hero. The studio had infamously been working on Duke Nukem Forever, the next flagship franchise entry, for over 12 years.

A final push to release Duke Nukem Forever began in 2007, marked by a short teaser starring the in-game hero that was released late that year.

As recent as January 2009, company steward George Broussard wrote on his Twitter feed that he was visiting Take-Two to show off the title.